This invention relates to a method of producing an anode of an electric cell by using a powdery active material and a binder.
In one type of organic electrolyte cells using a light metal such as lithium or sodium as the active material of the cathode, the active material of the anode can be selected from some kinds of compounds including metal oxides, metal halides, metal sulfides, silver chromate and graphite fluoride. To form any of such positive active materials into an anode body, usually it is necessary to mix a binder with the active material. Where the active material is low in conductivity it is also necessary to add a conductive material such as graphite, carbon black or a suitable metal powder.
In general, selection of a binder for forming an electrode body is an important factor in constructing an electric cell. In organic electrolyte cells of the aforementioned type, the binder for forming the anode body must be selected from limited polymers that are stable to the organic liquid used in the electrolyte, such as polyethylene, polypropylene and fluorocarbon resins such as polytetrafluoroethylene and copolymers of polytetrafluoroethylene with polyhexafluoropropylene. When any of these polymers in powder form is used as the binder, it is necessary for uniform dispersion of the binder in the active material and for realization of good formability that the binder occupies about 20% of the mixed material for the anode. Accordingly, if the mixed material contains 10% of a conductive material, the content of the positive active material becomes as low as about 70%. Furthermore, covering of the active material surfaces with such a large amount of binder results in a considerable decrease in electrochemically active surface areas of the anode and consequently in lowering of the cell characteristics.
For reducing such unfavorable phenomena and for improving the manner of mixing of the binder with the active material of the anode, it is prevailing to use a fluorocarbon resin in the form of an aqueous emulsion. For similar purposes, Japanese patent application primary publication No. 53-74242 (1978) proposes to use an aqueous dispersion of styrene-butadiene rubber as the binder and Japanese patent application primary publication No. 58-48360 (1983) an aqueous dispersion of nitrile-butadiene rubber. However, even when a binder of the aqueous emulsion or dispersion type is used still it is necessary that the binder occupies at least about 8 wt % of the anode for affording sufficient strength to the anode. It is desired to further decrease the amount of the binder without sacrificing the physical properties of the formed anode body.